using System;
public class Program
{
    public static void Main()
    {
        //int one = int.Parse(Console.ReadLine() ?? string.Empty);
        string[] num = (Console.ReadLine() ?? string.Empty).Trim().Split(' ');
        string[] age = (Console.ReadLine() ?? string.Empty).Trim().Split(' ');

        int family = int.Parse(num[2]); //家族の数
        int sum = int.Parse(num[0]) * int.Parse(num[1]);

        int[] fAge = new int[family];
        int need = 0;
        for (int i = 0; i< family; i++)
        {
            fAge[i] = int.Parse(age[i]);
            need += int.Parse(age[i]);
            if(sum <  need)
            {
                Console.WriteLine("-1");
            }
        }


        if(sum -  need > 0)
        {
            Console.WriteLine("{0}",sum-need);
        }


    }
}